Purveyors of prerecorded video tapes are distressed by the prevalence of counterfeiting but, prior to the present invention, there has been no convenient means of distinguishing between a counterfeit and a genuine recording. This problem is particularly vexing, because a video recording can be copied so easily and inexpensively onto blank tapes, the value of which may be a small fraction of that of the recording. The problem especially concerns purveyors of videocassettes that have been prerecorded with motion pictures, because a huge market has developed for their rental and sale.
Although I am unaware of any prior art that either deals with this particular problem or provides a magnetic recording video tape that would be counterfeit-resistant, British Patent Specification No. 1,331,604 (Pearce et al.) describes a magnetic recording medium comprising a magnetizable layer which includes a pattern of regions of magnetically anisotropic material that is oriented in a predetermined direction, while the balance of the layer is either unoriented or oriented at right angles to said predetermined direction. The pattern of regions provides a "magnetic watermark" which serves to identify and authenticate the medium that may take the form of a key, a credit card, passport, or the like. U.S. Pat. No. 4,104,513 (Pearce) describes a similar medium except that in areas outside the pattern of regions, the magnetically anisotropic material is oriented at angles other than right angles to the orientation inside the pattern of regions. Neither the Pearce et al. nor the Pearce medium would be useful for video recording.
U.S. Pat. No. 3,566,356 (Holm et al.) discloses a magnetic recording medium comprising magnetizable particles of differing H.sub.c which either may be combined into a single layer or may be in separate layers, e.g., a surface layer of gamma-Fe.sub.2 O.sub.3 particles and an under layer of barium ferrite particles having substantially higher H.sub.c than the gamma-Fe.sub.2 O.sub.3. The Holm patent discloses a sophisticated test for validating the medium, e.g., as a recordable stripe of a credit card, but does not suggest its use for video recording. Nor would the Holm medium be useful for video recording use unless the magnetizable particles of differing H.sub.c were in separate layers, and only those of a surface layer were of sufficiently low H.sub.c to be susceptible to video signals as well as the accompanying audio signals.
My U.S. Pat. Nos. 3,878,367; 3,986,205; and 4,090,662 concern magnetic recording media wherein, as in the Holm patent, the magnetizable particles are selected or oriented to provide validation of the media or documents to which the media are attached, e.g., a credit card. Only magnetic recording media of the latter two of my three patents could be commercially practical for video recording and then only when containing a surface magnetizable layer which is susceptible to video recording and an inner magnetizable layer which is not.
U.S. Pat. No. 3,219,353 (Prentky) discloses a magnetic recording medium for data storage having two magnetizable layers separated by a layer of non-magnetic shielding material. The coercivity (H.sub.c) of the outer layer is approximately 130 Oersteds and that of the inner layer is approximately 1000 Oersteds. The inner layer is recorded permanently with servo signals, and data signals can be recorded and read on the outer layer without disturbing or being disturbed by the servo signals. Although the patent does not say so, the servo signals could be a "magnetic watermark". U.S. Pat. No. 3,328,195 (May) discloses a magnetic recording medium similar to that of the Prentky patenet.
Video tapes, especially videocassette tapes of the "Betamax" and "VHS" formats, often have in addition to the magnetizable layer, an electrically conductive layer which serves to bleed off static charges. Usually the electrically conductive layer is on the backside of the tape and may be formulated to provide sufficient roughness and frictional characteristics for slip-free driving. A magnetic recording tape having such an electrically conductive backside layer is disclosed in U.S. Pat. No. 3,617,378 (Beck), although the patent does not mention video utility. The backside layer of the Beck patent includes nonmagnetic alpha-Fe.sub.2 O.sub.3 particles to impart the desired coefficient of friction.